This application claims the benefit of priority of German application No. 10127020.8 filed on Jun. 1, 2001, which is relied upon and expressly incorporated by reference herein.
The invention relates to a piston ring for use in a cylinder of an internal combustion engine. This piston ring is nitride-hardened at least on the sliding surface between piston ring and cylinder wall, wherein the nitride layer consists of a precipitated layer and a composite layer.
Among the thermo-chemical treatment methods, nitration or nitro-carburization (or carbonization) has long been used worldwide for structural components. Those thermochemical treatment methods provide improvement in the characteristics of the surface-layer with respect to wear resistance, corrosion resistance and endurance limit. In the field of automobile technology, these methods are mainly used for structural components subjected to high sliding stresses, e.g. for spur gears, gear shafts or piston rings.
During the nitriding, carbide and/or carbide/nitride modification, the carbon or nitrogen penetrates the surface of the components and interacts with the dislocations, thus leading to a change in the material characteristics. The dissolved, as well as the precipitated, nitrogen or carbon increase the hardness. Generally the higher the share of nitride-forming alloy elements in the component, the higher the hardness. Nitride layers can basically be divided into two layer regions, including a first diffusion layer and a second compound layer. For this, a porous rigid area develops in the outer region of the composite layer surface, which can negatively influence the operational behavior of the components stressed by sliding friction. With respect to the piston ring, this can lead to micro welding at the surface, to increased piston groove wear and to the formation of furrows on the cylinder wall.
German reference DE 35, 02, 143 C2 therefore proposes a piston-ring coating, in which a rigid composite layer is removed; and an oxide layer that positively influences the running-in behavior is additionally deposited onto the diffusion layer. The oxide layer in this case is soft enough, so that it can adapt easily to the cylinder wall at the start of operations, thus preventing the negative running-in phenomena that otherwise occur at the start. In addition, oxide layers have a good corrosion resistance. The claims for reference DE 35, 02, 143 C2 relate to a nitride-hardened high-alloyed steel, but do not take into account the fact that the nitride-hardening of high-alloyed steel negatively effects the corrosion resistance. The deposited composite layer is subsequently removed again, meaning that an additional operational step is required, and the rigid supporting layer on the piston ring is removed.
Reference DD 119, 822 describes depositing an oxide layer onto a nitride-hardened iron alloy to improve the corrosion resistance and, simultaneously, provide the surface with a decorative appearance, as well as create favorable conditions for accepting lubricants. In general, this reference deals with iron and iron alloys, in particular used for chip-removing tools, which are nitride-hardened and are subsequently oxidized in an oxygen-containing medium. Thus, an approximately 5 xcexcm thick oxidized cover layer is formed, which is advantageous for chip-removing tools. These 5 xcexcm thick cover layers, however, are unsuitable for components that are subjected to sliding friction since cracks can form in these layers. It is advantageous in this case if single-phase, oxidized cover layers with Fe3O4 are primarily formed. Even the composite layers are formed as single-phase layers, for example as Fe2-3N layers. However, formation of multi-phase oxidized cover layers or composite layers is not suggested in and does not follow from this reference.
Reference DE 195, 25, 182 C2 provides a general description of a method for creating corrosion-resistant layers and wear-resistant layers on materials with an iron base. With this method, areas close to the surface are enriched with nitrogen, carbon and oxygen. The process steps described therein include the steps of nitro-carburizing the material with iron base to form a compound layer consisting of iron carbo-nitrides (carbide/nitrides), of activating the surface of the material in a plasma-supported vacuum pressure process and of oxidizing the material to form a closed oxide layer. The references does not relate to using this method for components in internal combustion engines, in particular having low-alloyed piston rings.
Thus, it is the object of the invention to modify the above-described prior art and to develop a piston ring, which exhibits improved performance during its use, and the improved corrosion resistance and minimizing the wear on the piston-ring surface as well as in the piston ring groove.
This object is solved according to the invention by a piston ring for use in a cylinder of an internal combustion engine. The piston ring of the invention has a surface that is in sliding contact relationship with a wall of said cylinder during operation, and comprise
a piston ring which comprises iron or an iron alloy;
an oxide layer which is deposited on the outer surface of said piston ring and which is deposited over a compound layer, and which oxide layer, during operation is in sliding contact with said cylinder; and
a region of increased nitride content between said oxide layer and said iron or iron alloy dispose whereby the piston ring is nitride-hardened,
wherein the nitride layer comprises a precipitated layer and a compound layer, where said oxide layer is deposited over said compound layer.